1. Field of the Invention
The present invention relates to a conveying apparatus driven by a linear motor.
2. Background Art
A conveying apparatus driven by a linear motor is widely known. JP 2009-71955 A discloses an example of such a linear motor usable for the conveying apparatus. As shown in FIGS. 6A and 6B, a linear motor 100 includes primary magnetic poles 101 and secondary magnetic poles 102 arranged facing each other, and a cover 103 located between the magnetic poles 101 and 102. This cover 103 includes a mounting member 105 mounted on an inner wall 104 of a housing that constitutes the conveying apparatus, for example, as shown in FIG. 6B. The cover 103, for example, can allow the primary magnetic poles 101 on the fixed side to be located in the atmospheric environment, and the secondary magnetic poles 102 on the movable side to be located in a vacuum environment or a reduced pressure environment. This enables the conveying apparatus provided with the linear motor 100 to suitably convey an object in a vacuum environment or a reduced pressure environment, without releasing a gas through a plastic molded member around a coil in each primary magnetic pole 101 to the surroundings (vacuum environment), as in the case where the primary magnetic pole 101 is placed in vacuum environment.
In the linear motor 100 according to JP 2009-71955 A, a magnetic attraction force acts between the magnetic poles 101 and 102 facing each other. Further, a force to press the cover 103 toward the vacuum or reduced pressure side acts on the cover 103 due to the atmospheric pressure. The directions of the magnetic attraction force and a component of the force to press the cover 103 due to the atmospheric pressure which is directed toward the secondary magnetic poles 102 are coincident. Therefore, the magnetic attraction force and the component force may possibly be concentrated on a region of the inner wall 104 which is surrounded by the mounting member 105. Such concentration of forces may possibly cause a bending of the housing. As an influence of this bending, there may be cases where the distance (gap) between the magnetic poles 101 and 102 varies, thereby causing a change in thrust characteristics of the linear motor 100 (such as that the thrust becomes uneven), or each secondary magnetic pole 102 located on the movable side is prevented from moving smoothly relative to the primary magnetic pole 101 located on the fixed side by reasons such as application of a load to the bearing. As described above, it is a problem that the forces in the same direction are concentrated on the region of the inner wall 104 which is surrounded by the mounting member 105.
Therefore, it is rendered necessary to increase the rigidity of the housing including the region of the inner wall 104 which is surrounded by the mounting member 105. The rigidity is increased, for example, by increasing the plate thickness of the part of the housing which includes the aforementioned region, or adding a reinforcement structure thereto, which therefore cause an increase in cost as a matter of course.
Particularly, in the linear motor 100 according to JP 2009-71955 A, a facing surface 106 of the primary magnetic pole 101 to the secondary magnetic pole 102 is located parallel to the inner wall 104 of the housing, as shown in FIG. 6B. Here, it is assumed that the primary magnetic pole 101 is formed by winding a coil around a core formed by stacking a plurality of plate bodies, for example, as shown in FIG. 5B, and the stacking direction of the plate bodies of the core is coincident with the width direction of the primary magnetic pole 101 (the left-right direction in FIG. 5B and the up-down direction in FIG. 6B). Under this assumption, if engineers change the design of the magnetic poles 101 and 102 in attempts to increase the thrust of the linear motor 100, it is necessary to increase the number of the plate bodies to be stacked in the core. That is, it is necessary to increase the width dimension of each of the magnetic poles 101 and 102. The surface area of the cover 103 is increased by this increment of the width dimension, and therefore the force due to the atmospheric pressure is also increased. In addition to this, the increase in thrust of the linear motor 100 leads to an increase in magnetic attraction force between the magnetic poles 101 and 102. For these reasons, the possibility of bending of the housing is more increased, which makes the aforementioned problem more significant.